1. Field of the Invention
The present invention relates to a plasma process apparatus.
2. Description of the Related Art
As is known, active species such as ions, complex ions, and radicals are present in a plasma obtained by electrically discharging a certain type of gas, and surface treatment of a semiconductor wafer can be presented as one of the fields in which these active specifies are used. For example, a process performed by using a plasma allows process control with high precision. For this reason, in the manufacturing process of a semiconductor wafer, plasma processes are used to perform etching and formation of various films.
As one of methods of generating a plasma, a method of applying RF power to a process gas is available. A plasma process apparatus, e.g., a signal wafer processing type etching apparatus, using this method has a lower electrode disposed in a process chamber having an airtight seal structure, and an upper electrode serving as a gas supply portion and disposed to oppose the lower electrode. This apparatus is designed to generate a plasma by applying RF power between the upper and lower electrodes so as to etch the surface of an object to be processed, e.g., a semiconductor wafer, placed on the lower electrode.
In such a conventional plasma process apparatus, a coaxial cable is used as a means for connecting a lower electrode to an RF power supply. However, a cumbersome operation is required to connect a shielded line to a process chamber, and the impedance of the apparatus may be changed depending on the manner of connecting them. In addition, Teflon used as a dielectric member for a high-power coaxial cable is expensive. For these reasons, the use of an RF power supply rod having a double-pipe structure, in place of a coaxial cable, has been considered.
FIG. 1 shows a conventional etching apparatus using such an RF power supply rod. An upper electrode 11 also serving as a gas supply portion and a lower electrode 10 having a susceptor 13 supported on a susceptor support base 12 are disposed in a process chamber 1 to oppose each other. An inner conductive rod 14 of the RF power supply rod is inserted from the lower side of the process chamber 1 to extend to the susceptor 13, and an outer conductive pipe 15 of the RF power supply rod is connected to the bottom wall of the process chamber 1. This bottom wall is electrically connected to the upper electrode 11 via the side wall of the process chamber 1.
The lower end portions of the inner conductive rod 14 and the outer conductive pipe 15 are electrically connected to an RF power supply E and the ground via a matching circuit MC in a matching box M. The lower ends of the inner conductive rod 14 and the outer conductive pipe 15 are connected to the matching circuit MC via power supply rods 14a and 15a. Referring to FIG. 1, reference numeral 16 denotes an exhaust pipe; 17 and 18, insulating portions; and 19, a cooling reservoir to which a cooling medium is circulated/supplied, and reference symbol W denotes a wafer as an object to be processed.
Below the process chamber 1, there are a pipe for circulating/supplying a cooling medium such as liquid nitrogen to the refrigerant reservoir 19, a gas inlet pipe (not shown) for supplying a backside gas to the lower surface of the wafer W, and the like. Therefore, the matching box M is located at a considerably lower position from the bottom wall of the process chamber 1, and the RF power supply rod is considerably long. If the RF power supply rod is long, as described above, the axes of the inner conductive rod 14 and the outer conductive pipe 15 are sometimes greatly shifted from each other when they are coupled to the power supply rods 14a and 15a with screws or the like.
The sizes of wafers recently tend to increase from a conventional 6-inch wafer to 8- and 12-inch wafers. In addition, the liquid crystal panel techniques have advanced rapidly, and the sizes of panels tend to increase. Apparatuses for performing processes, e.g., film formation and etching, with respect to such LCD substrates have been developed.
When a plasma process, e.g., etching, is to be performed with respect to an object to be processed, e.g., a wafer or an LCD substrate having such a large size, a high voltage corresponding to an RF power of about 4 kW is required. In this case, in designing the apparatus, an RF power supply ratio must be set to have a predetermined characteristic impedance. Therefore, the ratio of the diameter of the inner conductive rod 14 to that of the outer conductive pipe 15 is determined on the basis of this characteristic impedance. For this reason, if the RF power increases, and the axes of the inner conductive rod 14 and the outer conductive pipe 15 are shifted and brought close to each other, a discharge may occur between the inner conductive rod 14 and the outer conductive pipe 15.
If a discharge has occurred in the RF power supply rod, the supply efficiency of RF power decreases, resulting in a decrease in etching rate. In addition, the plasma becomes unstable, and impedance matching cannot be performed. As a result, a stable plasma process cannot be performed. In addition, the RF power supply rod itself is damaged by the discharge.